The invention relates to a process for electrochemically determining the oxygen concentration. In particular in body fluids, by means of an oxygen sensor having a sensor electrode, and also to an oxygen sensor employing said process.
The measurement of oxygen concentration or of oxygen partial pressure is an important analytical problem. In medical technology, in particular, a rapid and accurate determination of the oxygen value is necessary. Thus, the determination of the oxygen in the blood of patients requires an accuracy of approximately 1 torr, in particular, in the range between 10 and 200 torr. At the same time, the drift in the signal in the course of three days, which essentially corresponds to the duration of measurement in the blood, should not exceed a value of 5 torr. The oxygen sensors hitherto used in medicine, however, fail to meet these requirements by far.
At present, use is generally made in medical technology of the Clark oxygen sensor (in this connection see U.S. Pat. Nos. 2,913,386, 3,260,656 and 4,076,596), which is based on the electrochemical principle. Although this sensor can be used in blood gas analyzers, it is not, however, suitable for determining the oxygen content of the blood since some important features of the sensor are an obstacle to an implantation or an operation in the body lasting a fairly long time. These include, in particular, a hydrophobic membrane, which is disposed in front of the measuring electrode. The properties of said membrane are in fact markedly altered by interactions in the body. In addition, the continuous and high oxygen consumption of the sensor results in a marked rejection reaction which further impedes the operation of the sensor.
Published European Patent Application 0 170 998 discloses a process for electrochemically determining the oxygen concentration, in particular, in biological material, by means of an oxygen sensor having a measuring electrode and a counterelectrode. In this process, two potentials are impressed cyclically on the measuring electrode, the first potential (measuring potential) being in the range between -1.4 and -0.4 V and the second potential (recovery potential) in the range between -0.2 and +0.2 V, referred in all cases to an Ag/AgCl reference electrode; in this connection, the length of stay at the measuring potential is small compared with the cycle time. The measuring signal evaluated in this process is the current flowing during the measurement period, in particular in the form of the charge. The known process is primarily used to measure relative variations in the oxygen concentration in order to match the frequency of a heart pacemaker to the patient's requirements.